No built in way to override new-pane actions right now, but `cmux --help` can automate all parts of cmux.

So you can make your own script that can make new panels/workspaces and just invoke it from the terminal:

  git worktree add -b my-branch ../repo-my-branch
  ws=$(cmux new-workspace 2>&1 | awk '{print $2}')
  cmux send --workspace "$ws" "cd ../repo-my-branch && claude"
  cmux send-key --workspace "$ws" Enter